Understanding ASHRAE 241: Healthier Indoor Air | Steril-Aire Blog

What is ASHRAE Standard 241?

ASHRAE Standard 241, Control of Infectious Aerosols, is the first code-enforceable indoor air quality (IAQ) standard designed to reduce the indoor transmission of infectious aerosols. Drawing on research and expertise gathered from the ASHRAE Epidemic Task Force, the new standard provides requirements for many air system design, installation, operation, and maintenance aspects.

In simplest terms, Standard 241 provides the minimum equivalent clean air requirements to reduce airborne diseases in commercial settings — such as offices, schools, and hospitals — and in residential settings — such as senior assisted-living facilities during periods of elevated risk.

“The requirements for filter and air cleaner testing incorporated in this standard go well beyond what is found in current standards,” the authors wrote in the foreword. “They are a major step in the direction of creating uniform and effective technology-agnostic criteria for characterizing filter and air-cleaner performance and safety.”


ASHRAE Standard 241 Prerequisite

Before implementing Standard 241, requirements for ASHRAE 62.1 must already be in place. In other words, you cannot apply ASHRAE 241 without first fulfilling the ventilation and IAQ requirements of ASHRAE 62.1.

The 2022 version of ANSI/ASHRAE 62.1 standard for Ventilation and Acceptable Indoor Air Quality defines the total outdoor air volume per person over time that must be brought into a building to dilute infectious aerosols.



What are the key aspects of ASHRAE Standard 241?

  1. Infection Risk Management Mode (IRMM):

The 241 Standard identifies additional layers of engineering controls to implement during periods of elevated risk of disease transmission. Typically documented in a building readiness plan, added measures to reduce infectious aerosol exposure are activated during IRMM operation.

It’s important to remember that AHJs (Authorities Having Jurisdiction), such as your health department, determine when to implement this infectious aerosol standard. Standard 241 can be applied at the discretion of a building’s management during periods of high viral loads, such as influenza or “flu” season.

  1. Requirements for Equivalent Clean Airflow Rate:

While ASHRAE 62.1 focuses on the amount of outdoor air required to dilute harmful pathogens, ASHRAE 241 concentrates on filtration and air cleaning and disinfection to control indoor air quality.

According to ASHRAE, “Standard 241 breaks new ground by setting requirements for equivalent clean airflow rate (ECAi), the flow rate of pathogen-free airflow into occupied areas of a building that would have the same effect as the total of outdoor air, filtration of indoor air, and air disinfection by technologies such as germicidal ultraviolet light (UV‑C).”

During the COVID-19 pandemic, commercial building engineers looked to ventilation rates identified in ASHRAE 62.1, which was never intended to mitigate the transmission of airborne diseases. ASHRAE 241 has now introduced targets for equivalent clean airflow rates, which buildings were not originally designed to achieve. The new ventilation requirements outlined in Standard 241 increase building ventilation rates by up to ten times that of current standards.



Equivalent Clean Airflow Calculator (ECAC)
The Equivalent Clean Airflow Calculator can help determine the existing system’s equivalent clean airflow for infection control as well as the modifications that achieve the target ECAi set by ASHRAE Standard 241. Download the ECAC

  1. Utilization of Filtration and Air Cleaning Technology:

Again, improving air quality is a cumulative goal. In other words, one can achieve the recommended 4-6 equivalent air changes by combining engineering controls, such as MERV 13 filtration, local HEPA filtration, outside air ventilation and upper-room GUV.

For the purposes of this standard, air cleaning refers to the reduction of infectious aerosols in the air.

Finally, a note about energy use. It’s often thought that buildings with higher IAQ levels expend much more energy.

Ironically, using higher levels of filtration and local air cleaning, such as disinfection with UV‑C, requires less energy – and is less carbon-intensive – than increasing outdoor air level volumes, previously considered the gold standard. Using outside air is neither sustainable nor affordable—and frankly, impossible, using the existing energy grid.

  1. Planning and Commissioning: 

Standard 241 provides assessment and planning requirements culminating in the development of a building readiness plan, a concept carried over from the work of the ASHRAE Epidemic Task Force. It also describes procedures for commissioning systems to determine their installed performance.

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